Thanks to everyone's contributions to this, both practical and tongue-in-cheek.
I can now report on an outcome. My hand was forced yesterday, with a last-minute request by the client to cover a community event featuring hundreds of people from a vast array of different national and ethnic backgrounds. Most of the shots would be candids. Speed of shooting was essential, and all had to work in the dreaded circle format.
Method on the day:
I found a plastic disc 20mm in diameter, and stuck a scrap piece of plastic to it as a handle, so I could apply pressure to it with a finger tip, and be able to run a 0.2mm pen around the circumference ( including under my finger ) without it moving. I practised about 10 times before I finally tried it on the screen. Despite what I'd thought, the grid lines on the screen are not easy to see when it is laid flat (Even tried putting it on top of a lightbox) so the accuracy of placement came down to an eye judgement of the disc being centred. As luck would have it, I got it pretty right. Spot on top to bottom, maybe a bee's dick out left to right. Through the viewfinder, it was perfect. A bit furry due to the nature of the pen, but fine enough to be the "constant reminder" I would need. It worked a charm on the job, I was composing and zooming to it in real time, giving me the greatest opportunity to get what I was after. I've processed the shots, and certainly no evidence of any disruption to focus or exposure.
Comments on other's suggestions:
Don, I did consider drawing the circle on acetate and placing it above the screen. My only concerns were the acetate moving when swinging the screen back into position, or heaven forbid, it perhaps preventing the screen from locking properly and dropping.... maybe half way throught the shoot. Although a screen has been sacrificed for the job, I felt that a safer way to go.
David, Your acetate overlay idea is a good one. Like you, I used to use them all the time in the "Old film days" on both 4x5 and Rollei to indicate a predetermined layout area. I'll make one up for the Camera LCD and also my Epson P-4500 for when the A.D. and I are doing mid-shoot appraisals during the main job later in the week.
Wisdom of hindsight - How I'd do it next time:
Trying to position the circling disc and holding it in place is a bit ham-fisted (especially with your holding finger in the way of drawing) and makes unintentional screen damage a concern. I think the following technique would be better:
1. Cut a frame which fits snugly around the screen and double-sided tape that frame to a clean flat surface. The frame sould be slightly thicker than the screen, so the screen sits below the surface of the frame.
2. Cut a circle out of thin card to the required size. Check the size circle that the pen gives relative to the card circle: You may need to make minor adjustments to compensate for the tip diameter that is in contact with the card relative to the tip that is doing the drawing. By making the frame in step 1 slightly thicker than the screen, this should ensure that the circle mask is not in contact with the screen during the drawing, and avoid any smudging, or abrasion from the surrounding mask. With regard to cutting the circle, circle cutters are available. I think most graphic designers/ art directors have them in their kit if you want to avoid buying one just for the job.
3. With the screen hold in position, you should be able to align the cut hole over it to line up with the grid marks, which should be far more visible than when using my original technique. Once it's right, fasten it by some means and draw the circle.
4. Obviously, the cut-out shape can be anything: oval, triangle, etc. depending on requirements.
The only other comments I would make is that I'm going to crop the full frame proofs before I give them to the client, probably to about a 3:4 proportion. The wasted space on either side of the full frame looks terrible ( to my eye, anyway), as though I haven't done my job properly and zoomed in.
Also, I might boost the file size a bit when I process the RAWs. The circle is only 36% of the frame, so the same percentage would apply to the 100% file size. At least the 1Ds2 is doing the best job possible for 35mm.
John.
